The chemistries for making photosensitive polymers for holographic applications have been demonstrated, at least to some degree, since approximately 1970. In fact, as a general principle a wide variety of photosensitive compositions may form a photosensitive polymer film that is be useful for making holograms. Photosensitive polymer films useful for holographic applications include ingredients such as a polymer carrier that may be a film-forming polymer, one or more photosensitive initiators, dyes or sensitizers, and different types of photoreactive compounds. To be useful for holographic storage applications, however, photosensitive polymer films must satisfy certain functional requirements.
An example of these requirements is that polymerization of the photosensitive polymer films during light exposure must be substantially completed in no more than a few hundred mJ/cm.sup.2. There is also a trade-off between film sensitivity and the material properties of the film. Although high sensitivity may yield higher grating intensity in a short period of time, high sensitivity of the photosensitive polymer film may affect other chemical and physical characteristics.
One characteristic to consider in forming a photosensitive polymer film is that the refractive index of the polymer carrier and that of the polymers newly formed by the reactive ingredients must be substantially different. Also, during polymerization, monomers must sufficiently diffuse from the dark fringes to the bright fringes of the film, while most other reactive species must stay in the bright zones of the film. Furthermore, noises created from scatter centers and arising from either electronic or morphological heterogeneities in the film must be significantly lower than is often obtainable.
Physical properties that govern the use of photosensitive polymers include the requirement that during and after polymerization the physical dimension of the films must remain largely unchanged. In addition, while photosensitive polymer films, on the one hand, must be sensitive to light, they must, on the other hand, be thermally stable at temperatures as high as 55.degree. C. for several years. Finally, in order to be useful for most applications, photosensitive polymers must store holograms fixed and stable under ambient conditions of 60.degree. C. and 95% relative humidity for at least 2000 hours.
Photosensitive polymer compositions that satisfy the above criteria may be classified according to the initiators used in the films. Two known classifications of polymers are (1) free radical initiator photosensitive polymers, and (2) cationic initiator photosensitive polymers. There are two types of free radical initiators: Type I and Type II. Type I initiators can generate free radicals upon absorption of photons or through excitation of exposed sensitizers. Examples of Type I free radical initiators are the following: ##STR1##
Upon absorption of photons or through excitation of sensitizers, the excited states of Type II photosensitive initiators, on the other hand, abstract an alpha hydrogen molecule from a tertiary amine. This generates free radicals for polymerization of vinyl or acrylates. Examples of Type II free radical initiators take the following form: ##STR2##
The above two types (i.e., Type I and Type II) of free radical initiators are useful for polymerization of acrylates and vinyl monomers. To maintain thermal stability, inhibitors are normally added to these monomers. Adding inhibitors helps to prevent premature thermal polymerization during film storage. In addition, a barrier film often covers the photosensitive polymer films. The barrier film prevents oxygen inhibition that may occur to the free radicals that arise from photoexposure of the film.
Because of the need to address these physical constraints, photosensitive polymer films that include free radical initiators are slow to respond to light exposure. The delays include an induction time delay or the time delay of a pre-pump stage, which must occur before it is possible to record on the film. In addition, during the pre-pump stage, pre-pump energy must generate sufficient free radicals to overcome pre-dissolved oxygen and inhibitors that exist in the films. Current commercial films of this type require more than 100 to 200 mJ/cm.sup.2 of pre-pump energy to even begin the recording process. As a practical matter, therefore, these films are not suitable for most holographic data storage applications.
Other types of photosensitive polymer films use cationic initiators. For example, since 1970 cationic initiators such as iodonium and sulfonium salts have been used to polymerize vinyl ethers, cyclic ethers and acetals and epoxies. These cationic initiators generate cationic species which are not sensitive to the oxygen. These cationic species, however, can be poisoned by active amino-hydrogen of the primary or secondary amino compounds such as the following: ##STR3##
When they are free from the above active hydrogen, the cationic species can have very long life and cause dark reactions. The long life cationic species are undesirable for making high density gratings in holographic application. This is because the cationic species can diffuse into dark fringes, reduce grating intensity or widen gratings over time. In addition, the cationic polymerization rate, in general, is very slow compared to the polymerization rate of the free radicals.
Since most traditional photosensitive initiators are not absorbing enough in the 685 nm.+-.5 nm ranges, dyes or photosensitizers are normally added all the above initiators. Dyes are chosen (1) if they have high absorption at 685 nm, (2) if they can be bleached by the presence of photosensitive initiators during exposure, and (3) if their quantum yield or efficiency of bleaching upon exposure is high.
Adding dyes produces yet another chemical consideration in photosensitive films. In general, the energy gap between the excited state of the dye and the excited state of the photo-initiators have to be in the order of few Kcal/Mol to provide an effective quantum yield.